Here are two straightforward sets of questions which deal with adding 8 to a single digit. Eventually children should know, off by heart, all the answers to these questions, but that takes a lot of time and practice. Confidence can be gained by learning some of them; eg knowing that 8 + 2 makes 10 can lead on to knowing that 8 + 3 makes 11.

Until they have been learnt most children will need to count on, perhaps using fingers or practical apparatus such as cubes to work the answers out.